Fossil soil has the dirt on early microbes.

Researchers looking for signs of ancient life have hit pay dirt in South Africa. Fossilized soil deposits there indicate that simple organisms may have colonized the continents as many as 2 billion years ago--a find that dramatically pushes back the history of life on land, report two South African geologists.

The record of ocean life reaches back 3.5 billion years, but firm evidence of life on the harsh, wind-scoured landscape does not appear in the first 3 billion years. "People say there was life in the oceans and shallow seas but no life on land [during that time]. We think we can give indirect evidence that there were some sort of algal mats or microbial colonies on land," says Jens Gutzmer of the Rand Afrikaans University in Auckland Park. Gutzmer and Nicolas J. Beukes describe their work in the March Geology.

The two scientists made their discovery in a manganese mine in the Gamagara rock formation of central South Africa. They noticed a hardened and compacted layer of ancient soil, several centimeters thick, that contained balls of hematite and pod-shaped masses of clay. Such structures are found today in a type of soil that forms in hot, humid, vegetation-rich environments.

With an estimated age of 2.0 to 2.2 billion years, the South African soil layer does not contain fossilized microbes, which are too fragile to survive in such rocks, the researchers say. Instead, it holds chemical clues to the presence of microbes in the past.

The top of the hardened soil layer appears bleached because it lacks red iron minerals, which were stripped from the upper layers and deposited in the lower ones. The researchers suggest that organic matter in the upper part of the soil reacted chemically with the iron, making it soluble. Rainwater then carried the iron into deeper layers.

The source of the organic matter was bacteria, algae, or other simple organisms inhabiting the soil and the ground surface, the researchers suggest.

The oldest widely accepted fossils of land organisms hail from the Ordovician period, 475 million years ago. Several scientists have reported circumstantial evidence of earlier land life but have yet to convince most researchers.

The South African find comes as welcome news for Gregory J. Retallack, a paleobotanist at the University of Oregon in Eugene, who contends that life on Earth may have originated in soils and later spread into the oceans. "Life on land, I think, has had big effects all throughout Earth history," says Retallack.

Other researchers view the South African finding as muddy. "It's a teaser," says Heinrich D. Holland, who studies ancient soils at Harvard University. "It could be true, but I consider it a weak straw in a strong wind."

Holland notes that swamps covered parts of the South African landscape around the time these soils formed. Gutzmer and Beukes agree that the organic matter responsible for the iron leaching could have come from aquatic organisms in these bodies of water.

For conclusive evidence of early land life, researchers will need to find fossils of ancient microbes, says Holland.